A diverse fauna of fossil vertebrates is reported from the Dogie Locality, an early Tertiary (Paleocene) site in the lower Tornillo Formation in Big Bend National Park, Texas. Remains of fish, amphibians, turtles, lizards, snakes, crocodiles, and mammals were recovered by screen washing of sediments. The occurrence of mammal species previously known from classic early Paleocene localities indicates a Puercan age (Pu2 or Pu3) for this locality. To date Dogie has yielded the largest mammalian fauna of early Paleocene age in the Big Bend area.

INTRODUCTION

Early Tertiary continental sediments containing vertebrate fossils are preserved within a graben in the Big Bend area of southwest Texas. The sequence of Late Cretaceous/early Tertiary sediments exposed in the area has been interpreted as the southernmost Laramide-age intermontane basin (the Tornillo Basin) of the North American cordillera (Lehman, 1991). It lies approximately 850 kilometers southeast of the San Juan Basin of New Mexico. Terrestrial deposits of the Tornillo Formation extend from the lower, intergradational boundary with the Aguja Formation upward to the base of the Chisos Formation, which consists predominantly of volcanic deposits. The Javelina Member of the Tornillo Formation spans the Cretaceous\Tertiary boundary (Standhardt, 1986; Schiebout et al., 1988). Maastrichtian dinosaurs (Lawson, 1976) and the giant pterosaur Quetzalcoatlus northropi (Lawson, 1975) occur in the lower Javelina Member of the Tornillo Formation. Lawson (1972) also first reported mammalian fossils from the upper Javelina Member of the Tornillo Formation, from a locality known as Tom's Top, which has since been recognized as early Paleocene in age, probably from the same time period as Dogie (Standhardt, 1986).

This report is preliminary. More detailed descriptions of fossil finds and formal descriptions of new taxa are in preparation.

THE LOCALITY AND ITS SETTING

The Dogie locality, in the northwestern part of Big Bend National Park (Figure 1), was discovered by Robert Rainey (Davies, personal comm. 1983). Fossils occur in light gray and pink bentonitic mudstones with interbedded sandy siltstones lenses that contain limonitic and gypsiferous concretions. Some chalcedony is present in the sediments, and gypsum and authigenic clays are common. From 1983 to 1988, numerous fossils were recovered from surface prospecting and from screen washing 3.3 metric tons of sediment. The degree of preservation of individual fossils is extremely varied. Many very fragile, small bones are silicified and opalized; other fossils are less well-preserved, and some appear to have partially digested. Coprolites are common. Some creep of the soft sediments is evident on the low hills, which are undermined by pseudokarst that has formed deep holes. The locality is stratigraphically 73 meters above the highest occurrence of dinosaurian remains in the local section (Figure 2), and 88 meters above a locality which has yielded teeth of Cretaceous mammals.

VERTEBRATE FAUNA

Fish remains recovered from Dogie include teeth of a new species of freshwater ray (Dasyatis), bones and teeth of bowfins (Amia), gars (Atractosteus) and indeterminate teleosts. Amphibians are represented by frog limb bones. Abundant turtle remains have been found, which indicate the presence of several types of turtles. Glyptosaurine lizards are represented by teeth and scutes. Snakes were also present, as shown by vertebrae of the boid Dunnophis and of a new blind snake (Scolecophidia, new genus and species). This is the earliest known occurrence of blind snakes in the fossil record. Both piercing and crushing crocodilian teeth are very common.

Mammalian fossils from Dogie include teeth of new forms as well as of species known from other deposits yielding fossils of early Paleocene age, particularly from the San Juan Basin, New Mexico. Dogie mammals already known from other areas include the multituberculate Mesodma thompsoni, the periptychid condylarths Carsioptychus coarctatus and Haploconus inopinatus, and the mioclaenid condylarth Bomburia prisca.

Dogie new mammalian taxa representing genera known from other areas include new species of the multituberculates Ptilodus, Cimolodon, and Viridomys, and possibly new species of the marsupial Peratherium, the palaeoryctid Gelastops, the carnivore Protictis, and the condylarth Baioconodon. In addition, several teeth of a new genus and species of mioclaenid condylarth have been recovered.

CONCLUSIONS

The Dogie fauna at Big Bend most closely resembles Puercan faunas of the San Juan Basin of New Mexico, as might be expected from their relative proximity. On the basis of the presence of Bomburia prisca and Carsioptychus coarctatus the Dogie locality is assigned a late Puercan age. As no remains have yet been recovered of Taeniolabis taoensis, exact placement in either the Pu2 or Pu3 interval-zone (Archibald et al., 1987) cannot be made.

The occurrence of a new species of Viridomys, which was previously known only from the early Campanian of Canada (Fox, 1971), represents a significant range extension for this genus. Range extensions are also indicated for Haploconus inopinatus, previously reported from the Torrejonian Dragon Canyon local fauna (Gazin, 1941), Gelastops, and Protictis. The appearance of Peratherium at Dogie is markedly earlier than previously known. The southerly position of the Big Bend region may have contributed to the establishment of a biotic refugium for forms such as Viridomys. The region could also have been a source area for peripatric speciation. The discoveries at Dogie show that a significant southern area can be added to the list of North American sites containing faunas spanning the Cretaceous\Tertiary boundary.

ACKNOWLEDGEMENTS

This study was conducted under National Science Foundation Grant EAR-8216488. Support was also provided by the Louisiana State University Museum of Geoscience, the Atlantic Richfield Foundation, the Shell Companies Foundation, Chevron, the American Association of University Women, Edith W. Standhardt, and Judith A. Schiebout. I thank Judith A. Schiebout, Leigh Van Valen, and Earl Manning for helpful comments. J.A. Wilson, Wann Langston, Kyle Davies, and Melissa Winans provided access to specimens and locality information. Officials of Big Bend National Park have been particularly helpful. The expert piloting of J.C. Short enabled the recovery of material for screen washing. Alvin Phillips, Tom LeFebvre, and students from Louisiana State University and Sul Ross State University provided essential field and laboratory assistance.